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Thread: Diesels - Common Rail compared to Direct Injection

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    Regular Member Innes's Avatar
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    Default Diesels - Common Rail compared to Direct Injection

    Hi Folks, I dont know much about fuel systems so thought some one could enlighten me.

    Why is Common Rail more efficent and seems to produce more power than Direct Injection. How do both methods of fuel delivery work?

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    Regular Member big rich's Avatar
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    google it

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    Regular Member rushy's Avatar
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    Direct injection injects directly into the cylinders and from what i'm aware its more efficient?

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    Regular Member padawun's Avatar
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    Common rail, does exactly what it says on the tin. There is a fuel rail that is "common" to all the cylinders that is held under high pressure, the injectors
    are pulsed by a control module in the same way as a petrol injector. The reason that they are more efficient, is due to the accuracy of the fuel being fired in to the cylinder by the injector and secondly the fact the common rail fule systems run at a much higher fuel pressure.

    These systems are great until they go wrong, high pressue pumps and peizo encrypted injectors are both causing concern. To be euro 4 complient, the system will be fitted with a DPF in the exhaust. This again may prove to be costly and fragile whilst in service.

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    Regular Member john_k_sri's Avatar
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    Euro 4 in practice did not require DPF's across the range, as the Euro-4 compliant CDTI's came out in 2004 without them but VX fitted them on models with higher emissions (such as Autos, Estates and Signums). With Euro 5 they've become mandatory.

    It was Euro5 that signalled DPF's across all diesels, as seen in the Insignia, New Astra, and New Meriva.

    Common-rail injection has become commonplace on diesels, whereas direct-injection on petrols has not.
    Last edited by john_k_sri; 3rd October 2010 at 00:36.

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    Full Member GazVXLINE170's Avatar
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    Most common rail diesels are direct. The common rail means the fuel is readily available.

    Gazza4
    Insignia VX-Line Nav CDTI 170.

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    Regular Member Innes's Avatar
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    Quote Originally Posted by padawun View Post
    Common rail, does exactly what it says on the tin. There is a fuel rail that is "common" to all the cylinders that is held under high pressure, the injectors
    are pulsed by a control module in the same way as a petrol injector. The reason that they are more efficient, is due to the accuracy of the fuel being fired in to the cylinder by the injector and secondly the fact the common rail fule systems run at a much higher fuel pressure.

    These systems are great until they go wrong, high pressue pumps and peizo encrypted injectors are both causing concern. To be euro 4 complient, the system will be fitted with a DPF in the exhaust. This again may prove to be costly and fragile whilst in service.
    Another stupid question.. Why doesnt petrol engines use common rail?

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    Regular Member padawun's Avatar
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    Quote Originally Posted by Innes View Post
    Another stupid question.. Why doesnt petrol engines use common rail?
    By definition they do, the petrol supply to the injectors is via a fuel rail (often refered to as a fuel manifold) this holds the fuel at the regulated pressure and the injectors normally mount directly to this rail. Excess fuel is returned to the tank by a fuel return pipe via a fuel pressure regulator.

    Make sense?

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    Regular Member FatboySteve's Avatar
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    On the subject, can someone explain why a common rail diesel fuel pump still needs to be timed up ? Surely if it's just supplying pressure, and the actual injection is an electronically operated valve, the timing of it wouldn't matter, eg petrol fuel pumps, mechanical power steering pumps etc

    I understand why the older ones had to be, because they were proper mechanical pumps, so if not timed up correctly the diesel doesn't hit the cylinders at the right time, but don't get why that is still needed.


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    Regular Member the-wizard's Avatar
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    They dont!, the cdti pump does'nt either.

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